1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical pickup device which can obtain a stable reproduction signal by increasing the numerical aperture of an objective lens.
An optical pickup records and reproduces information such as video or audio data onto/from optical recording media, e.g., discs. The structure of a disc is such that a recorded surface is formed on a substrate made of plastic or glass. In order to read or write information from a high-density disc, the diameter of the optical spot must be very small. To this end, the numerical aperture (NA) of an objective lens is generally made large and a light source having a shorter wavelength is used. Using the shorter wavelength light source and large numerical aperture, however, reduces a tilt allowance of the disc with respect to the optical axis. The thus-reduced disc tilt allowance can be increased by reducing the thickness of the disc.
Assuming that the tilt angle of the disc is .theta., the magnitude of a coma aberration coefficient W.sub.31 can be obtained from: ##EQU1## where d and n represent the thickness and refractive index of the disc, respectively. As understood from the above relationship, the coma aberration coefficient is proportional to the cube of the numerical aperture. Therefore, considering that the numerical aperture of the objective lens required for a conventional compact disc is 0.45 and that for a digital video disc is 0.6, a digital video disc has a coma aberration coefficient of about 2.34 times that of a compact disc having the same thickness. The maximum tilt allowance of the digital video disc is therefore controlled to be reduced to about half that of the conventional compact disc. Accordingly, in order to assimilate the maximum tilt allowance of the digital video disc to that of the compact disc, the thickness of the digital video disc should be reduced.
However, such a thickness-reduced disc adopting a shorter wavelength (high density) light source, e.g., a digital video disc, cannot be used in a conventional recording/reproducing apparatus, e.g., a disc drive for the compact disc adopting a longer wavelength light source, because a disc having a non-standard thickness (digital video disc) results in a spherical aberration corresponding to the difference in disc thickness from that of a normal disc (compact disc). If the spherical aberration is greatly increased, the spot formed on the disc cannot have the light intensity necessary for recording, which prevents accurate recording of the information. Also, during reproduction, the signal-to-noise ratio is too low to reproduce the information accurately.
Therefore, an optical pickup adopting a light source having a short wavelength, e.g., 650 nm, which is compatible for discs having different thicknesses, such as a compact disc or digital video disc, is necessary.
2. Description of the Related Art
For the above purpose, research into apparatuses which can reproduce and record information from/onto two kinds of discs having different thicknesses with a single optical pickup device adopting a shorter wavelength light source is underway. Lens devices respectively adopting a hologram lens and a refractive lens have been proposed (Japanese Patent Laid-open Publication No. hei 7-98431).
FIGS. 1 and 2 show the focusing of zero-order and first-order-diffracted light onto discs 3a and 3b having different thicknesses, respectively. A hologram lens 1, provided with a lattice pattern 11, and a refractive objective lens 2 are provided along the light path in front of discs 3a and 3b. The lattice pattern 11 diffracts light beams 4 from a light source (not shown) passing through the hologram lens 1, to thereby separate the passing light into first-order-diffracted light 41 and zero-order light 40 each of which is focused with a different intensity by objective lens 2 for the appropriate focus point on the thicker disc 3b or the thinner disc 3a, and thus enable data read/write operations with respect to discs having different thicknesses.
However, in using such a lens device, the separation of the light into two beams (i.e., the zero-order and first-order light) by hologram lens 1 lowers the utilizing efficiency of the actually regenerated light to about 15%. Also, during a read operation, the information is included in only one of the two beams, and the beam carrying no information is likely to be detected as noise. Moreover, the fabrication of such a hologram lens requires a high-precision process for etching a fine hologram pattern, which increases manufacturing costs.
FIG. 3 is a schematic diagram of a conventional optical pickup device (U.S. Pat. No. 5,281,797) which, in lieu of using a hologram lens as above, includes an aperture diaphragm 1a for changing the aperture diameter, so that data can be recorded onto a longer wavelength disc as well as a shorter wavelength disc and so that information can be reproduced therefrom. An aperture diaphragm 1a is installed between an objective lens 2 facing a disc 3 and a collimating lens 5 and controls a light beam 4 emitted from a light source 9 and transmitted through a beam splitter 6, by appropriately adjusting the area of the light beam passing region, i.e., the numerical aperture. The diametrical aperture of the aperture diaphragm 1a is adjusted in accordance with the focused spot size on the disc being employed and always passes the light beam 4a of the central region but passes or blocks the light beam 4b of the peripheral region in accordance with the adjusted state thereof. In FIG. 3, a reference numeral 7 denotes a focusing lens and reference numeral 8 denotes a photodetector.
In the optical device having the above configuration, if the variable diaphragm is formed by a mechanical diaphragm, its structural resonance characteristics change depending on the diaphragm's effective aperture, and thus installation onto an actuator for driving the objective lens becomes difficult in practice. To solve this problem, liquid crystal may be used for forming the diaphragm. This, however, greatly impedes the miniaturization of the system, deteriorates heat-resistance and endurance, and increases manufacturing costs.
Alternatively, a separate objective lens for each disc may be provided so that a specific objective lens is used for a specific disc. In this case, however, since a driving apparatus is needed for replacing lenses, the configuration becomes complex and manufacturing costs increase accordingly.